1. Field of the Invention
The present invention relates to a liquid discharger for successively pushing out liquid inside a tube by successively pressing and squashing a portion of the tube, and an apparatus including the liquid discharger.
2. Description of the Related Art
A liquid discharger (tube pump) for discharging liquid inside a resilient tube by successively pressing and squashing the tube has been conventionally known.
For example, there is, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-110712, a liquid discharger for sending fluid into a tube by pushing out a plurality of tube pusher members disposed along the tube by a cam shaft and successively squashing the tube. The cam shaft of the liquid discharger is driven by a spring through a train of wheels.
In addition, there is, as disclosed in Japanese Unexamined Patent Application Publication No. 5-69558, a liquid discharger of a type that successively squashes a tube by biasing a pressure roller by a compressing spring.
Further, there is a liquid discharger having a structure in which a tube is disposed in the form of an arc or a semicircle and the top surface of the tube is pressed and squashed by a circular cylindrical roller.
Such related liquid dischargers have the following problems.
In the liquid discharger in which a plurality of tube pusher members are pushed out by a cam shaft, friction is produced between the cam shaft and the tube pusher members, so that energy loss becomes large, and the cam shaft and the tube pusher members are worn by the friction, thereby giving rise to the problem that durability cannot be increased. In particular, in this liquid discharger, rotational motion of the cam shaft is converted into advancing and retreating movement of the tube pusher members with respect to the tube, and a large force needs to be exerted to squash the tube by the tube pusher members. Therefore, friction is produced between the cam shaft and the tube pusher members, so that there is a problem in that the cam shaft and the tube pusher members are worn.
At least three tube pusher members are required. In order to achieve smoother discharging of liquid, many tubes of the order of eight tubes are required. Since friction is produced between the many tube pusher members and the cam shaft, a large force is required to drive the cam shaft and to squash the tube using the tube pusher members. Therefore, for example, a large motor must be provided, thereby making it difficult to reduce the size of the liquid discharger.
Even in the liquid discharger using a pressure roller, the area of contact between the pressure roller and the tube is large, so that a large force is required to squash the tube. Therefore, a large motor is required to drive the pressure roller, thereby making it impossible to reduce the size of the liquid discharger. In order to rotatably mount the presser roller, a subassembly for, for example, previously securing a roller bearing or the like to a guide roller is required. Therefore, there are problems in that the size of the liquid discharger is increased and that costs are increased. Further, since a large friction is produced due to a large area of contact between the pressure roller and the tube, when the liquid discharger is used for a long period of time, wearing due to friction occurs, thereby making it impossible to increase durability of the liquid discharger.
Further, even in the liquid discharger in which the tube is pressed and squashed by a circular cylindrical roller, since the area of contact between the roller and the tube is large, a large motor is required for driving the roller. In addition, since slipping occurs due to a difference between the speeds of movement of the inside surface (surface closer to the center of the arc or semicircle formed by the tube) and outside surface of the roller, friction loss occurs. To overcome this problem, the roller may be formed with a conical shape.
When the conical roller is used, it is necessary to consider the direction in which the conical roller is set. For example, when the tube is disposed in a circular form, it is necessary to dispose the axis of rotation of the conical roller so as to face the center of the circular form of the tube. Also, when the conical roller is used, in order to sufficiently press and squash the tube, it is necessary to set the surface where the tube is provided and the surface where the roller presses and squashes the tube parallel to each other. When variations occur in, for example, an assembly operation, it becomes difficult to maintain these surfaces parallel to each other, so that the pressing and squashing operation becomes unstable. Therefore, when the conical roller is used, the assembly operation must be precisely performed by considering the setting direction, thereby making the assembly operation troublesome to carry out.
As described above, these related liquid dischargers have a first problem in that it is difficult to increase durability, to reduce size, and to make it easy to perform an assembly operation.
A liquid discharger which successively presses and squashes a tube is such that, even while it is not operating, at least a portion of the tube is pressed and squashed all the time. In particular, during the period of time from the time after the assembly of the liquid discharger at a plant is completed to the time the user starts to use the liquid discharger, a force is exerted only on a portion of the tube for a long period of time. As a result, the tube undergoes plastic deformation, so that its capacity is changed. Therefore, even if the user starts to use the liquid discharger, an error in the discharge rate from the liquid discharger may occur, thereby giving rise to a second problem in that it is difficult to reduce errors in the discharge rate.
When the tube is rubbed and pulled by a ball (that is, when the ball moves on the tube while it presses and squashes the tube), the tube is stretched or its resiliency is reduced, so that variations in discharge rate may occur. In particular, at the initial stage immediately after the user starts using the liquid discharger, the tube with a length close to its natural length is pulled when it is rubbed and pulled, so that the inside diameter of the tube changes, as a result of which errors in the rate of discharge tend to be large. Therefore, when the rate of discharge is to be precisely controlled, it is necessary to perform a test run, thereby giving rise to a third problem in that it is difficult to increase work efficiency.